CN113639829A

CN113639829A - Method for monitoring liquid level of vehicle liquid tank, storage medium, control unit and SCR system

Info

Publication number: CN113639829A
Application number: CN202010391754.XA
Authority: CN
Inventors: 刘毅
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2020-05-11
Filing date: 2020-05-11
Publication date: 2021-11-12
Also published as: EP3910299B1; KR102521131B1; KR20210137930A; EP3910299A1

Abstract

The present application relates to a method for monitoring a liquid level in a vehicle liquid tank, comprising the steps of: causing a first ultrasonic sensor to emit ultrasonic waves and receive a first echo signal to detect a liquid level in the liquid tank; causing the second ultrasonic sensor to transmit ultrasonic waves and receive a second echo signal when the first ultrasonic sensor does not receive the first echo signal for a period of time; and judging whether the liquid tank is empty or not based on the second echo signal. The application also relates to a machine-readable non-volatile storage medium, a control unit and a vehicle SCR system. According to the liquid level detection method and device, whether the liquid tank is emptied or not can be reliably judged when the first ultrasonic sensor cannot detect the liquid level of the liquid tank.

Description

Method for monitoring liquid level of vehicle liquid tank, storage medium, control unit and SCR system

Technical Field

The present application relates to a technique for monitoring the level of liquid in a tank, in particular an exhaust gas treatment tank, of a vehicle, in particular a diesel vehicle.

Background

When operated, the engine of a vehicle produces exhaust gases having a high content of nitrogen oxides, which cannot be directly emitted to the atmosphere, but need to be treated by a Selective Catalytic Reduction (SCR) system before being emitted to the atmosphere. For diesel vehicles, one effective technique for treating exhaust gas emitted from an engine is to reduce the content of nitrogen oxides in the exhaust gas by an SCR method. The SCR method treats exhaust gas by using an exhaust gas treating fluid (usually an aqueous urea solution) in an SCR system to convert harmful nitrogen oxides in the exhaust gas into harmless nitrogen and water vapor, thereby reducing harmful gas emissions of an engine.

An SCR system generally comprises a tank for containing an exhaust gas treatment liquid, an injection device for injecting the exhaust gas treatment liquid into the exhaust pipe, and a delivery device for drawing the exhaust gas treatment liquid from the tank and supplying it under pressure to the injection device. An ultrasonic sensor is arranged in the liquid tank and used for detecting the liquid level of the tail gas treatment liquid. The ultrasonic sensor realizes liquid level measurement by means of an interface between the tail gas treatment liquid and air above the tail gas treatment liquid. However, when the liquid tank is excessively filled with the exhaust gas treatment liquid, the air in the liquid tank is completely squeezed out, an interface between the exhaust gas treatment liquid and the air does not exist, no ultrasonic wave is reflected back to the ultrasonic sensor, and therefore the ultrasonic sensor cannot detect the liquid level. In addition, when the exhaust gas treatment liquid in the liquid tank is excessively consumed or emptied, the liquid level may fall below the ultrasonic wave emitting portion of the ultrasonic sensor, and the liquid level may not be detected by the ultrasonic sensor. Using only such a level sensor cannot determine whether the tank is empty.

Disclosure of Invention

In view of the above-described problems, the present application aims to provide a method for monitoring the liquid level in a tank, in particular an exhaust gas treatment tank, of a vehicle, in particular a diesel vehicle, to reliably determine whether the tank is empty.

According to one aspect of the present application, there is provided a method for monitoring a liquid level in a vehicle liquid tank having a first ultrasonic sensor and a second ultrasonic sensor mounted on a bottom of the liquid tank, the first ultrasonic sensor for detecting the liquid level in the liquid tank by emitting ultrasonic waves toward a top of the liquid tank and receiving an echo signal, the second ultrasonic sensor for judging whether or not liquid is present below an ultrasonic wave emitting portion of the first ultrasonic sensor by emitting ultrasonic waves along the bottom and receiving an echo signal, the method comprising the steps of:

causing the first ultrasonic sensor to emit ultrasonic waves and receive a first echo signal to detect a liquid level in the liquid tank;

causing the second ultrasonic sensor to transmit ultrasonic waves and receive a second echo signal when the first ultrasonic sensor does not receive the first echo signal for a period of time;

and judging whether the liquid tank is an empty tank or not based on the second echo signal.

According to another aspect of the present application, there is provided a machine-readable non-volatile storage medium having stored thereon program instructions for implementing the foregoing method.

According to yet another aspect of the application, a control unit is provided having a memory and a processor, the memory storing executable program instructions that when executed cause the processor to implement the aforementioned method.

According to yet another aspect of the present application, there is provided a vehicle SCR system, comprising a tank for containing an exhaust treatment liquid, an injection device for injecting the exhaust treatment liquid into an exhaust pipe, a delivery device for drawing the exhaust treatment liquid from the tank and pressure-supplying it to the injection device, and the aforementioned control unit.

The method for monitoring the liquid level in a liquid tank, in particular in an exhaust gas treatment liquid tank, of a vehicle, in particular a diesel vehicle, can determine whether the liquid tank is emptied when the ultrasonic liquid level sensor does not detect the liquid level of the liquid tank. In addition, the method can also reliably judge which liquid filling state the liquid tank is in: 1) the liquid tank is an empty tank; 2) the tank is overfilled; 3) the liquid level in the liquid tank is reduced to the lower part of the ultrasonic wave transmitting part of the ultrasonic wave liquid level sensor, so that different reactions are made according to different liquid filling conditions, and the normal operation of a vehicle system is ensured.

Drawings

The above-described and other aspects of the present application will be more fully understood and appreciated in view of the accompanying drawings. It should be noted that the figures are merely schematic and are not drawn to scale. In the drawings:

FIG. 1 is a schematic illustration of an SCR system according to a preferred embodiment of the present application; and

FIG. 2 is a flow chart of a method for monitoring tank liquid level according to a preferred embodiment of the present application.

Detailed Description

Preferred embodiments of the present application are described in detail below with reference to examples. It should be understood by those skilled in the art that these exemplary embodiments are not meant to limit the present application in any way.

Fig. 1 schematically shows an exhaust SCR system for a vehicle, in particular a diesel vehicle, according to a preferred embodiment of the present application, associated with an exhaust pipe (not shown) of an engine of the vehicle for injecting an exhaust treatment fluid, typically an aqueous urea solution, into the exhaust pipe for reducing the content of nitrogen oxides in the exhaust gas. The SCR system may comprise a tank 1 for containing an exhaust gas treatment liquid, an injection device 3 for injecting the exhaust gas treatment liquid into the exhaust pipe, and a delivery device 5 for drawing the exhaust gas treatment liquid from the tank 1 and supplying it under pressure to the injection device 3. The delivery device 5 may comprise a supply pump 7 and a back-suction pump 9. When the SCR system is operating, the supply pump 7 in the delivery device 5 draws off the exhaust treatment liquid from the tank 1 through the suction line 11, pressurizes it and supplies it to the injection device 3 through the supply line 13. Subsequently, the injection device 3 atomizes the exhaust gas treatment liquid and injects it quantitatively into the exhaust pipe. When the vehicle is stopped, the back-suction pump 9 in the delivery device 5 performs back-suction to return the exhaust gas treatment liquid to the liquid tank 1 through the return line 15 to prevent the exhaust gas treatment liquid from being deposited, crystallized, etc. in the injection device 3 and the delivery device 5.

With continued reference to fig. 1, the SCR system may further include a first ultrasonic sensor 17, a heater 19, a second ultrasonic sensor 25, and a reflector 27 disposed within the tank 1. The first ultrasonic sensor 17 may be used to detect the level of the off-gas treatment liquid in the tank 1. The heater 19 may be used to heat the off-gas treatment liquid in the tank 1. The SCR system may also comprise a control unit 20 associated with the tank 1, the first ultrasonic sensor 17, the heater 19, the second ultrasonic sensor 25, the injection device 3, the delivery device 5 and other functional elements of the SCR system for controlling their operation and monitoring their status. The control unit 20 may be, for example, an ECU of a vehicle.

A first ultrasonic sensor 17 is mounted in the tank 1 at the bottom 21 and is operable to detect the level of the off-gas treatment liquid in the tank 1 by transmitting ultrasonic waves towards the top 23 of the tank 1 and receiving echo signals. Specifically, the first ultrasonic sensor 17 achieves liquid level detection by emitting ultrasonic waves to the interface between the off-gas treatment liquid in the tank 1 and the air above and receiving an echo signal reflected by the interface. The control unit 20 may determine the level of the off-gas treatment liquid in the tank 1 by comparing the echo signal with a predetermined reference value. However, when the tank 1 is excessively filled with the exhaust gas treatment liquid, the air in the tank 1 is completely pushed out, an interface between the exhaust gas treatment liquid and the air does not exist, no ultrasonic wave is reflected back to the first ultrasonic sensor 17, and thus the first ultrasonic sensor 17 does not detect the liquid level. Further, when the exhaust gas treatment liquid in the liquid tank 1 is excessively consumed or emptied (for example, manually emptied or leaked due to breakage), the liquid level falls below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, no ultrasonic wave is reflected back to the first ultrasonic sensor 17, and the first ultrasonic sensor 17 therefore cannot detect the liquid level. It is impossible to judge whether the liquid tank 1 is emptied using only the first ultrasonic sensor 17. In other words, when the liquid level is not detected by the first ultrasonic sensor 17, it is impossible to determine whether the liquid tank 1 is empty. For example, when it is necessary to determine the state of liquid filling in the liquid tank 1 at the time of vehicle start, if the first ultrasonic sensor 17 does not detect the liquid level, it is not possible to determine whether the liquid tank 1 is empty. When the liquid level is not detected by the first ultrasonic sensor 17, it is determined whether the liquid tank 1 is empty. As described above, the reason for the evacuation may be that the tank 1 is manually evacuated or leaks due to breakage. In this case, it is necessary to judge whether the liquid tank 1 is empty, and if it is determined that the liquid tank 1 is empty, it is necessary to prompt the user to check the integrity of the liquid tank.

The control unit 20 of the present application may determine whether the liquid tank 1 is empty (may be referred to as a first filling condition) when the liquid level is not detected by the first ultrasonic sensor 17. The control unit 20 needs to use the second ultrasonic sensor 25 when determining whether the liquid tank 1 is in the first filling condition. The second ultrasonic sensor 25 is mounted in the bottom 21 of the tank 1 and is operable to determine whether or not the exhaust gas treating liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor 17 by emitting ultrasonic waves along the bottom 21 of the tank 1 and receiving an echo signal, thereby determining whether or not the tank 1 is empty. The second ultrasonic sensor 25 may be disposed at a position lower than the ultrasonic wave emitting position of the first ultrasonic sensor 17. Preferably, the second ultrasonic sensor 25 may be disposed at the lowest position of the bottom 21 of the liquid tank 1. The reflector 27 may be arranged facing the second ultrasonic sensor 25, and the second ultrasonic sensor 25 may be used to determine the type of medium (e.g. gas and/or liquid) present between the second ultrasonic sensor 25 and the reflector 27 by transmitting ultrasonic waves towards the reflector 27 and receiving echo signals reflected by the reflector 27. Specifically, it is possible to judge whether or not the exhaust gas treatment liquid is present between the second ultrasonic sensor 25 and the reflector 27 by comparing the echo signal with a predetermined reference value. This makes it possible to determine whether or not the exhaust gas treatment liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, thereby determining whether or not the liquid tank 1 is in the first liquid-filled condition, i.e., whether or not the liquid tank 1 is empty. In one example, the reflector 27 is a plate extending from the bottom 21 of the tank 1. In another example, the reflector 27 may be a plate extending from a side wall of the tank 1.

When the control unit 20 finds that the first ultrasonic sensor 17 does not detect the liquid level within a certain period of time (for example, about 300 seconds, but may be more than 300 seconds or less than 300 seconds), it activates the second ultrasonic sensor 25 to emit ultrasonic waves toward the reflector 27 and receive an echo signal in order to determine whether the tank 1 is in the first filling condition. Specifically, it is possible to judge whether or not the exhaust gas treatment liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor 17 by comparing the echo signal of the second ultrasonic sensor 25 with a predetermined reference value. When it is determined that there is no exhaust gas treatment liquid below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, it may be determined that the liquid tank 1 is in the first liquid-filled condition, i.e., the liquid tank 1 is empty. However, if it is determined that the exhaust gas treatment liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, there are two possible situations: 1) the tank 1 is overfilled (which may be referred to as a second fill condition); or 2) the liquid level in the tank 1 has fallen below the ultrasonic emission site of the first ultrasonic sensor 17 but has not been emptied (which may be referred to as a third fill condition).

In this case, the control unit 20 needs to further determine whether the tank 1 is in the second or third fill condition by operation of the heater 19. Specifically, the heater 19 has different operating (heating) states, in particular, different temperature rising rates, operating currents, and the like between the presence and absence of the exhaust gas treatment liquid in the surroundings. For example, when the exhaust gas treatment liquid is present around the heater 19, the temperature increase rate of the heater 19 is slow (steady heating), and when the exhaust gas treatment liquid is not present around the heater 19, the temperature increase rate of the heater 19 is rapidly increased (non-steady heating).

The heater 19 may be, for example, a PTC (positive temperature coefficient) heater, the heating element of which is made of a PTC material, the resistance of which increases with temperature. With this material, when the exhaust gas treatment liquid is raised to a certain temperature, the resistance of the heating element is raised and the current is reduced, so that the power of the heater 19 is reduced, thereby preventing the exhaust gas treatment liquid from being overheated, and thus the heater 19 has a self-temperature limiting function. When the exhaust gas treatment liquid is not present around the heater 19, the temperature rise rate of the PTC heater 19 is very fast, and the operating current rapidly decreases.

The control unit 20 determines whether the tank 1 is in the second charging condition or the third charging condition based on the heating state of the heater 19. For this reason, the heater 19 is disposed at a position higher than the ultrasonic wave emitting portion of the first ultrasonic sensor 17. For example, the heater 19 is mounted on the side wall of the tank 1.

The control unit 20 starts the heater 19 or maintains the heating state of the heater 19 and monitors the heating state of the heater 19 to judge whether the liquid tank 1 is in the second liquid filling condition or the third liquid filling condition when the liquid level of the liquid tank 1 needs to be further judged to be in the second liquid filling condition or the third liquid filling condition, namely when the liquid level cannot be detected by the first ultrasonic sensor 17 and the exhaust treatment liquid is determined to be present below the ultrasonic emission part of the first ultrasonic sensor 17 by the second ultrasonic sensor 25. Specifically, in the case where the control unit 20 detects that the heater 19 is heating steadily, it may be determined that the tank 1 is in the second filling condition, and in the case of non-steady heating, it may be determined that the tank 1 is in the third filling condition. For example, in a situation where the heater 19 is heating steadily, the heater 19 will rise normally, thereby determining that there is also an exhaust treatment liquid level above the ultrasound emitting portion of the first ultrasound sensor 17, and thus determining that the tank 1 is overfilled (second filling situation). In the case where the heater 19 is not constantly heating, the heater 19 is too fast, and it is determined that the liquid level in the tank 1 has fallen below the ultrasonic wave emitting portion of the first ultrasonic sensor 17 but has not been emptied (third liquid-filled state). In the case where the heater 19 is a PTC heater, the control unit 20 detects the operating current of the heater 19, and if the operating current falls at a normal rate, the heater 19 is deemed to be heating-stable, and a determination is made that the tank 1 is in the second fill condition, and if the operating current falls at an excessive rate or falls below a predetermined minimum current value, the heater 19 is deemed to be heating-unstable, and a determination is made that the tank 1 is in the third fill condition.

When the control unit 20 determines that the tank 1 is in the first level condition, i.e. the tank 1 is empty, the control unit 20 may issue an alarm to alert the user to check the tank 1. For SCR systems with automatic priming functionality, the control unit 20 may temporarily disable this functionality. If the user checks that the liquid tank 1 is intact, the automatic liquid filling function can be started again or manual liquid filling can be carried out; if the user finds the tank 1 broken, the entire tank 1 needs to be replaced. When the control unit 20 determines that the tank 1 is in the second level condition, i.e. the tank 1 is overfilled, the control unit 20 may alert the user that sufficient off-gas treatment liquid is available in the tank 1 and maintain the heating state of the heater 19 (e.g. at a lower temperature). When the control unit 20 determines that the tank 1 is in a third level condition, i.e. the liquid level in the tank 1 has fallen below the ultrasonic emission location of the first ultrasonic sensor 17 but is not being emptied, the control unit 20 may turn off the heater 19 and may issue an alarm to alert the user to replenish the tank 1 with off-gas treatment liquid. For SCR systems with automatic filling function, the control unit 20 may initiate the filling operation of the tank 1.

The SCR system of the present application can reliably determine whether the liquid tank 1 is emptied when the first ultrasonic sensor 17 cannot detect the liquid level of the liquid tank 1. Further, the above-described technique can reliably determine which of the following filling conditions the liquid tank 1 is in: 1) the liquid tank 1 is an empty tank; 2) the liquid tank 1 is overfilled; 3) the liquid level in the tank 1 falls below the ultrasonic emission part of the first ultrasonic sensor 17, so that different reactions are made according to different liquid filling conditions, and the normal operation of a vehicle system is ensured.

It will be appreciated that various modifications may be made by those skilled in the art to the structure and function of the SCR system, and in particular the control unit 20, described hereinbefore within the scope of the present application. In addition, in other embodiments, when the exhaust gas treatment liquid is present between the second ultrasonic sensor 25 and the reflector 27, the concentration of the exhaust gas treatment liquid may be further determined according to the echo signal of the second ultrasonic sensor 25.

The present application also relates to a method for monitoring the liquid level in a tank, which can optionally be applied in the SCR system described above and can be implemented by means of the control unit 20 described above. One possible flow of the method is shown schematically in fig. 2.

As shown in fig. 2, in step S1, the first ultrasonic sensor 17 is caused to emit ultrasonic waves toward the top 23 of the liquid tank 1 and to receive a first echo signal to detect the liquid level in the liquid tank 1.

Next, in step S2, it is determined that the first ultrasonic sensor 17 has not received the first echo signal for a certain period of time (for example, about 300 seconds).

Next, at step S3, the second ultrasonic sensor 25 is caused to emit ultrasonic waves along the bottom 21 of the tank 1 and receive a second echo signal.

Next, at step S4, it is determined whether or not the exhaust gas treatment liquid is present below the ultrasonic wave emitting portion of the first ultrasonic sensor 17 based on the second echo signal to determine whether or not the liquid tank 1 is an empty tank.

If it is determined in step S4 that there is no exhaust gas treatment liquid below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, it is determined that the liquid tank 1 is an empty tank, then it goes to step S5.

At step S5, an alarm is issued to remind the user to check the tank 1.

On the other hand, if it is determined in step S4 that the exhaust gas treatment liquid exists below the ultrasonic wave emitting portion of the first ultrasonic sensor 17, it is determined that the liquid tank 1 is not emptied, it goes to step S6.

In step S6, the heater 19 in the liquid tank 1 is activated or the heating state of the heater 19 is maintained, and the heating state of the heater 19 is monitored.

Next, in step S7, it is determined whether or not the heating state of the heater 19 is in the steady heating. The determination is achieved, for example, by whether the temperature rise rate of the heater 19 exceeds a predetermined temperature rise rate, whether the operation current decrease rate of the heater 19 exceeds a predetermined current decrease rate, or whether the operation current value of the heater 19 is lower than a predetermined minimum current value.

If it is determined in step S7 that the operating state of the heater 19 is in steady heating (for example, the temperature increase speed does not exceed the predetermined temperature increase speed, the operating current decrease speed does not exceed the predetermined current decrease speed, or the operating current value of the heater is not lower than the predetermined minimum current value), it is determined that the liquid tank 1 is being overfilled, it goes to step S8.

In step S8, the user is alerted that sufficient exhaust gas treatment liquid is available in the tank 1 and the heating state of the heater 19 can be maintained (for example, when the temperature is low).

On the other hand, if it is determined in step S7 that the operating state of the heater 19 is in unstable heating (for example, the temperature increase speed exceeds the predetermined temperature increase speed, the operating current decrease speed exceeds the predetermined current decrease speed, or the operating current value of the heater 19 is lower than the predetermined minimum current value), it is determined that the liquid level in the liquid tank 1 has fallen below the ultrasonic wave emitting portion of the first ultrasonic sensor 17 but has not been evacuated, it goes to step S9.

In step S9, the heater 19 is turned off, and an alarm is issued to remind the user to replenish the tank 1 with the exhaust gas treatment liquid. For an SCR system with automatic charging function, the charging operation of the tank 1 may be initiated.

In addition, in order to avoid the misdetection, other steps may also be performed before performing steps S1 to S9. For example, it is checked whether the first ultrasonic sensor 17, the second ultrasonic sensor 25, and the heater 19 are powered on and/or an error is reported before performing steps S1 to S9. In addition, when the liquid in the liquid tank 1 is solidified due to a low temperature (for example, the freezing point of the urea aqueous solution is-11 ℃), the first ultrasonic sensor 17 cannot detect the liquid level either. Therefore, the temperature in the liquid tank 1 can be detected before steps S1 to S9 are performed. If the temperature in the tank 1 is below the freezing point, the heater 19 may be activated to heat first, followed by performing steps S1 to S9.

It should be further appreciated that the method as illustrated in FIG. 2 may be executable program instructions that are stored on a machine-readable non-volatile storage medium. It will also be appreciated that the control unit 20 may have a memory and a processor, wherein the memory may store executable program instructions that when executed cause the processor to implement the method shown in fig. 2.

It is to be noted that the foregoing describes an exhaust SCR system for vehicles, in particular diesel vehicles, a control unit thereof and a related method, which are capable of determining whether the tank 1 is empty when the ultrasonic level sensor does not detect the level of the tank. Further, the above-described technique can reliably determine which of the following filling conditions the liquid tank 1 is in: 1) the liquid tank 1 is an empty tank; 2) the liquid tank 1 is overfilled; 3) the liquid level in the liquid tank 1 is reduced to the lower part of the ultrasonic wave transmitting part of the ultrasonic wave liquid level sensor, so that different reactions are made according to different liquid filling conditions, and the normal operation of a vehicle system is ensured. The scope of the present application thus covers a variety of situations where a fill condition is determined based on an ultrasonic sensor and possibly a heater in the vehicle fluid tank.

It should also be understood that the first ultrasonic sensor 17 and the second ultrasonic sensor 25 may be the same sensor. In this case, the sensor may be made to simultaneously function as the first ultrasonic sensor 17 and the second ultrasonic sensor 25 by means of an ultrasonic diverter.

The present application is described in detail above with reference to specific embodiments. It is to be understood that both the foregoing description and the embodiments shown in the drawings are to be considered exemplary and not restrictive of the application. It will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit of the application, and these changes and modifications do not depart from the scope of the application.

Claims

1. A method for monitoring the liquid level in a vehicle liquid tank (1), said tank (1) having a first ultrasonic sensor (17) and a second ultrasonic sensor (25) mounted at the bottom (21) of said tank (1), said first ultrasonic sensor (17) being adapted to detect the liquid level in said tank (1) by emitting ultrasonic waves towards the top (23) of said tank (1) and receiving echo signals, said second ultrasonic sensor (25) being adapted to determine whether liquid is present or not below the ultrasonic emission location of said first ultrasonic sensor (17) by emitting ultrasonic waves along said bottom (21) and receiving echo signals, said method comprising the steps of:

-causing the first ultrasonic sensor (17) to emit ultrasonic waves and to receive a first echo signal to detect the level of liquid in the tank (1);

causing the second ultrasonic sensor (25) to emit ultrasonic waves and receive a second echo signal when the first ultrasonic sensor (17) does not receive the first echo signal for a period of time;

and judging whether the liquid tank (1) is empty or not based on the second echo signal.

2. The method of claim 1, further comprising the steps of:

determining that the liquid tank (1) is empty when it is determined that there is no liquid below the ultrasonic wave emission site of the first ultrasonic sensor (17) based on the second echo signal.

3. The method according to claim 1, characterized in that the tank (1) further comprises a heater (19) for heating the liquid in the tank (1), the heater (19) being arranged at a position higher than an ultrasonic wave emitting position of the first ultrasonic sensor (17);

the method further comprises the steps of:

when it is determined that liquid is present below the ultrasonic wave emission site of the first ultrasonic sensor (17) based on the second echo signal, the heater (19) is activated or maintained to heat the liquid in the liquid tank (1), and the heating state of the heater (19) is monitored.

4. A method according to claim 3, characterized in that the method further comprises monitoring the heating state of the heater (19) on the basis of the heating temperature of the heater (19), determining that the heater (19) is in an unstable heating state when the rate of rise of the heating temperature exceeds a predetermined temperature rise rate value, and/or when the heating temperature exceeds a predetermined temperature value, and determining that the heater (19) is in an unstable heating state when the rate of rise of the heating temperature does not exceed a predetermined temperature rise rate value, and/or when the heating temperature does not exceed a predetermined temperature value.

5. The method according to claim 3, further comprising monitoring the heating state of the heater (19) based on a change in the operating current of the heater (19), determining that the heater (19) is in the non-steady heating state when a rate of change in the operating current exceeds a predetermined current change rate value, or the operating current reaches a predetermined limit current value, and determining that the heater (19) is in the steady heating state when the rate of change in the operating current does not exceed a predetermined current change rate value, or the operating current does not reach a predetermined limit current value.

6. The method according to any one of claims 3 to 5, characterized by determining that the tank (1) is overfilled when it is determined that the heating state of the heater (19) is in steady heating.

7. The method according to any one of claims 3 to 5, characterized in that when it is determined that the heating state of the heater (19) is in unsteady heating, it is determined that the liquid level in the tank (1) falls below the ultrasonic wave emitting portion of the first ultrasonic sensor (17) but is not evacuated.

8. The method according to claim 1, characterized in that the tank (1) is an exhaust gas treatment tank in a vehicle SCR system, and the liquid in the tank (1) is an exhaust gas treatment liquid.

9. A machine-readable non-volatile storage medium having stored thereon program instructions to implement the method of any of claims 1 to 8.

10. A control unit (20), the control unit (20) having a memory and a processor, the memory storing executable program instructions which, when executed, cause the processor to carry out the method of any one of claims 1 to 8.

11. A vehicle SCR system comprising a tank (1) for containing an exhaust gas treatment liquid, an injection device (3) for injecting the exhaust gas treatment liquid into an exhaust pipe, a conveying device (5) for drawing the exhaust gas treatment liquid from the tank (1) and pressure-supplying it to the injection device (3), and a control unit (20) according to claim 10.